Lignocellulosic materials such as wood are the raw materials used for the production of pulps and papers. In order to make papers, lignocellulosic materials such as wood are first reduced to pulps (discrete fibres) by a mechanical or chemical pulping process. In mechanical pulping, pulps such as the so-called thermomechanical pulp (TMP) and chemithermomechanical pulp (CTMP) are produced (with retention of lignin) mainly through the action of mechanical forces in a yield of ≧90. Following the pulping process, bleaching of the pulps to a whiter colour is often carried out prior to the process of papermaking. The whiteness of pulps and papers is commonly estimated by ISO (International Standardization Organization) brightness determination that measures the directional reflectance of light at 457 nm of the papers in an Elrepho instrument [TAPPI Test Methods, T452 om-92, Tappi Press: Atlanta, 1996]. A low ISO brightness such as 30% indicates deep brown papers and a high ISO brightness such as 85% represents white papers. Unbleached and bleached mechanical pulps typically have ISO brightness values of 45-65% and 70-85%, respectively, depending on the wood species, the pulping and bleaching conditions. The majority of lighin present in wood remains in unbleached and bleached mechanical pulps. This lignin reduces the pulp brightness at the time of its production and causes further brightness loss by a process referred to as colour reversion or yellowing when mechanical pulp or paper produced from mechanical pulp is exposed to light and/or heat. Colour reversion limits the application of mechanical pulps to products designed to have a short life such as newspaper while more lignin-free chemical pulps are used for long-life products in various grades.
Over the past fifty years or so, many methods have been reported for the inhibition of colour reversion of mechanical pulps and papers. These methods involve either a chemical treatment on pulps aimed at the modification of the yellowing-causing lignin or a surface treatment on papers made from mechanical pulps with yellowing inhibitors such as an UV absorber [Heitner, in: Photochemistry of Lignocellulosic Materials, ed. Heitner and Scaiano, p. 192-204, ACS Symposium Series 531, ACS 1993].
PCT, WO 99/05108, of Ciba Specialty Chemicals and published Feb. 4, 1999 and U.S. Pat. No. 6,254,724B1 issued to Ciba Specialty Chemicals on Jul. 3, 2001 describe the use of hindered nitroxide radicals, particularly derivatives of 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (4-hydroxy-TEMPO) as effective yellowing inhibitors alone or when combined with an UV absorber and other coadditives. All the examples provided in these publications involve syringe-injecting/spraying of an inhibitor solution to already-formed paper sheets, similar to other paper surface treatment methods reported elsewhere in the prior art. Although the descriptions are broad, indicating that the extremely broad class of inhibitors can be added to pulps, pulp slurries and papers, at all possible manufacturing stages, there is no teaching of how the inhibitors might be successfullly applied to pulp slurries and retained by the pulps. From these publications, it is not evident at all how the inhibitors, particularly those water-soluble ones, can be retained by pulp fibres on pulp slurries in aqueous media. The actual teachings are of coating the paper sheets with the inhibitors. It has recently been concluded that the inhibitor system based on hindered nitroxide radicals has no affinity for pulp fibres and thus must be applied to the surface of already-formed papers, in the press or dryer section of the paper machine, or on an off-line coater [Yuan et al., J. Pulp Paper Sci. 28(5:159-166, 2002].
U.S. Pat. No. 4,178,861 discloses a process for the delignification of lignocellulosic material which consists of pre-treating the material with a quinone or hydroquinone compound in a liquor consisting of an amine delignifying liquor, wherein the amine can be a polyalkylene polyamine such as triethylenetetramine. The amine is used as a base to remove lignin from the lignocellulosic material at elevated temperature (170° C.). U.S. Pat. No. 5,641,385 describes the use of ethyleneamines, including triethylenetetramine and other oligomers of ethylene amine to wash pulp to remove lignin. U.S. Pat. No. 6,200,938 B 1 discloses amphoteric derivatives of aliphatic polyamines such as triethylenetetramine with long chain fatty acids, esters or triglycerides as effective softening compositions for paper and other consumer and industrial products. None of the amines or amine derivatives described in these publications has any yellowing inhibitors or hindered amine light stabilizers attached to them. In addition, there is not any improvement on the light-stability of the lignocellulosic material treated with such amines or amine derivatives.
U.S. Pat. No. 5,318,851 describes epoxy resins containing bound, light-stabilizing groups. The hindered amine light stabilizer disclosed contains a hydrazido group (R—C(O)NR′—NHR″) that allows the attachment of the light stabilizer to the epoxy resins. Epoxy resins have completely different chemical, physical and morphological properties than lignocellulosic materials.
U.S. Pat. No. 6,416,627 B1 issued to Ciba Specialty Chemicals on Jul. 9, 2002 discloses polymeric stabilizers with high affinity to pulp. Although the descriptions are broad, indicating that an extremely broad class of polymeric stabilizers with pendant hindered nitroxide, hydroxylamine or hydroxyl-ammonium salts can be added at various points in the paper-making process, there is no teaching of how the polymeric stabilizers might be successfully applied to pulp slurries containing various dispersed and/or dissolved components/chemicals, and be retained by the pulps. The actual teachings are of applying the inhibitors to paper sheets from an aqueous solution or from 1:1 ethanol/dioxane solution. In addition, no polymeric stabilizers with pendant hindered amine are described. This patent does not teach creating fibre-reactivity for the stabilizers by grafting them onto an oligomeric amine compound bearing multiple cationic charges.
U.S. Pat. No. 6,447,644 B1 issued to Ciba Specialty Chemicals on Sep. 10, 2002 describes inhibition of pulp and paper yellowing using nitroxides, hydroxylamines, and other coadditives such as UV absorbers. The composition of the claimed nitroxides and hydroxylamines includes those possessing one quaternary ammonium (—N+R1R2R3) group on the side-chain of the molecules. Compounds with a single positively charged group such as an ammonium group may form an ionic pair with the negatively charged sites of wood pulp fibres and might be retained on the fibres during the filtration of the fibrous suspension from deionized water. In papermaking, the fibrous suspension is dispersed in the so-called white water, which contains a considerable concentration of ions of elements such as calcium, magnesium, aluminum, or sodium. These mineral cations will compete with any compounds that contain a single positive charge and will cause their poor retention on the fibril.
PCT, WO 02/25007 A2 published on Mar. 28, 2002 describes a method for the production of light-stable lignocellulosic materials, in particular, the production of mechanical wood pulps with improved light stability, as well as the resulting pulps of improved light stability and papers containing such pulps. The method involves the reaction, in an aqueous medium, of the materials with a water-soluble, fibre-reactive yellowing inhibitor possessing a primary or secondary amino (—NH2 or —NHR) functional group, or the reaction of a water-soluble, fibre-reactive, hindered amine light stabilizer possessing a primary or secondary amino (—NH2 or —NHR) functional group in an alkaline peroxide bleaching medium or in an aqueous medium with a subsequent bleaching of the materials in an alkaline peroxide bleaching medium.
In the process of papermaking, pulps such as bleached mechanical pulps are first mixed with various papermaking additives and water to form pulp slurry called a furnish or a stock. The water used to form the pulp slurry consists of fresh water and circulating process water (also called white water). Fresh water and particularly white water may contain various dispersed components/chemicals such as particulate fibrous particles (also called fines) or calcium carbonate (CaCO3) and dissolved components/chemicals such as calcium ions (Ca2+). The high concentration of dissolved components/chemicals can cause detachment of additives that are originally bound to the pulp fibres. It is important that any yellowing inhibitors attached to mechanical pulps will be stable and remain attached to the pulps during pulp processing and papermaking in an aqueous medium containing various dispersed and/or dissolved components/chemicals, and in particular multivalent ions such as Ca2+.
Reductive amination of aldehydes or ketones with ammonia, primary or secondary amines, i.e. reductive alkylation of ammonia, primary or secondary amines with aldehydes or ketones, in the presence of hydrogen and a hydrogenation catalyst, is an economical and effective way to obtain simple amino compounds [March, in: Advanced Organic Chemistry, John Wiley & Sons: New York, p. 798-800, 1985]. Sodium triacetoxyborohydride has also been used in the laboratory to affect the reductive amination [Abdel-Magid, et al., J. Org. Chem. 61: 3849-3862, 1996].
Hindered nitroxide (>N—O.) compounds such as 4-oxo-2,2,6,6-tetramethylpiperidine-N-oxyl (4-oxo-TEMPO) and 4-amino-2,2,6,6-tetramethylpiperidine-N-oxyl (4-amino-TEMPO) have deep orange colour and are paramagnetic due to the presence of the nitroxide radical. Proton nuclear magnetic resonance (1H NMR) characterization of hindered nitroxide compounds is done by reduction of the nitroxide to its diamagnetic, hydroxylamine (>N—OH) derivative with a reducing agent such as sodium dithionite [Ozinskas and Bobst, Helv. Chim. Aata 63: 1407-1411, 1980] or by conversion of the nitroxide to its diamagnetic, hydroxylamine hydrochloride (>N+(H)OHCl−) derivative with aqueous hydrochloric acid in ethanol [Sosnovsky and Cai, J. Org. Chem. 60: 3414-3418, 1995]. Conversion of the nitroxide to its hydroxylamine or hydroxylamine hydrochloride derivative also removes its orange colour.